ANOTHER jetting question

[m in sc]

that flatspot is an age old issue, and its cured differently on each bike

[JKV45]

I'm not too hip on drilling the jets because now you don't know what size you have and the number stamped on them is wrong. How about going to a local dealer, they normally have jets. Bikes with chambers may not pull well at 5000 rpm in high gear. Have you ridden other RD's that seem to run better?

I believe the internal shape of the jet is not straight-through, and when the shape gets altered the smooth flow does as well.

[gargantua350]

I got rid of the flat spot completely and other problems associated converting to cheap flat slides. But again, that was my application.

[m in sc]

its straight with a lead in bevel. either way, drilling is bad

[RDDave]

Although I am not fluent in fluid dynamics, I understand the problems associated with drilling jets. I have done it for decades when I don't have the correct jets. But ONLY for experimenting. By drilling, I can determine if it wants more fuel. And by inserting wires, the opposite. Again, as quick experiments on which way to go and, to some extent, how far. 

Now, it pulls strong.......in first, second and third. But starts 'surging' in 4th and gets worse in every upper gear. I have 180 jets and it seems to like the needle all the way up. I am finally getting some color in the plugs.

It also seems to like a lot of timing; as in about 2.0. It just doesn't pull as hard with it at 1.8 or so. And the surging also seems to take a bigger toll with slower timing. I have not tried more timing. I always run 93 in my 2 strokes. And I haven't heard any noises that I would subscribe to spark knock. Nor have I seen anything on the plugs to indicate too much lead.

So, what's with the 'surge'? It is not missing. It is a 'surge'. And has been there. I was hoping the new ignition would have dealt with it, but it hasn't. And I wished that I had just ditched the points (as was REPEATEDLY suggested) and went with this ignition right off. But, I didn't. 

Appreciate, as always, any input. Thank You!

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[m in sc]

sure you don't have a fuel feed issue? might be sucking the bowls dry in the higher gears. check gas tank cap vent and petcock flow.
also, its about throttle position for tuning.  seems like you're describing a feed issue.

the difference between 1.8 and 2.0 is really negligible, 1.8 actually gives better top end just slightly as the higher 2 strokes go the more they like the timing backed off

also drilling 2t jets in a mikuni direct feed circuit is a lot different than a car carburator, but for testing ... i get it. but its also totally different results.

[RDDave]

I thought about the possibility of a fuel feed issue, too. So, I tested it today and I have 6 ounces flow in 30 seconds through each line to the carb. Seems good.

I also checked the right carb flow into a jar. Looked okay. I haven't checked the left carb for flow.

I agree that it sounds like fuel flow. Any suggestions on further checks other than pulling the left carb and seeing how it flows into a jar?

I will try leaving the gas cap loose. I certainly have dealt with that on my old cars more than once. I should have already done that, but haven't. I guess I thought that the flow check I did on each line would have shown a problem with air lock. But I will test the flow again tomorrow and let it run longer to see if it starts to reduce flow.

Thanks again.

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[m in sc]

flow test it longer,  sometimes it takes a while to show itself. pinch one line off, then flow the other. reason being it can displace air from the other line (suck air).

btw i aced fluids in college 🤪